U.S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a direct printing apparatus. The direct printing apparatus includes a rotatable cylinder or toner carrier, which retains electrically charged toner or printing particles on its outer periphery, and a backing electrode spaced apart from the toner carrier. The backing electrode is electrically connected to a power source, thereby forming an electric field that attracts the charged toner particles on the toner carrier toward the backing electrode. Interposed between the toner carrier and the backing electrode is an insulative plate that includes a plurality of apertures through which the toner particles can pass. A pair of signal and base electrodes surrounds each aperture. The signal electrode is mounted on one surface, adjacent to the backing electrode, of the insulative plate and the base electrode is mounted on the other surface, adjacent to the toner carrier, of the insulative plate.
With the direct printing apparatus, when an image signal is applied to the signal and base electrodes, the toner particles on the toner carrier, opposing to those electrodes, are energized and then propelled through associated apertures onto the sheet substrate, forming an image corresponding to the image signal on the sheet substrate.
An amount of toner particles being energized and then propelled depends upon the Coulomb's force that is inversely proportional to the square of the distance between the toner carrier and backing electrode. When a rotatable cylindrical member is employed as the toner carrier, due to its curvature, a distance between one portion of the cylindrical member and the backing electrode is different from a distance between another portion of the cylindrical member, spaced away in a peripheral direction from the one portion and the backing electrode. This results in that the amount of toner particles moving into the aperture in a closest region where a distance between the toner carrier and the backing electrode is minimized differs from the that moving into another aperture in another region spaced away in a rotational direction of the cylindrical member from the closest region, thereby degrading a quality of the resultant image.
Likewise, if a distance between one end of the cylindrical member and the backing electrode (or the insulative plate) is different from that between the opposite end of the cylindrical member and the backing electrode (or the insulative plate), a density of an image formed at or adjacent to one end of the cylindrical member is not identical to that formed at or adjacent to the other end of the cylindrical member, further degrading the quality of the resultant image.